ScreamingMonkey is the project to add script-engine integration glue to Tamarin, so that it can handle
tags in other browsers, starting with IE (using ActiveScript interfaces).
A functional implementation exists in the tamarin-central Mercurial repository (look for an axscript directory). A number of problems remain - some due to limitations in the Scripting Engine, others related to work that still needs to be done on Tamarin itself - see the tracking bug for up-to-date information, but the best thing is to download the binary and try it out!
A binary is available here - it is a typical Windows installer, including uninstaller, and includes the engine, a README and some test files (md5sum of installer is c1fb03a8393102b642635fdcfb4e201d.) Although it should be considered an Alpha release, it works well enough for many purposes.
We are currently planning Stage 3 - see Tamarin:ScreamingMonkey:Planning notes for what remains.
You can usually find markh and brendan on irc.mozilla.org #developers or #tamarin. If you want to help and have the hacking, testing, documenting, or evangelising skills, just ping us (the Tamarin:ScreamingMonkey:Planning notes may offer some ideas about how you can help)